Then a few months ago I fell, and apparently broke my tailbone (coccyx).
It still hurts.
This causes me to sit forward to relieve the pressure.
This has been straining my back. Which has reawakened the 2004 injury.
When I sit back to relieve THAT strain, I'm putting pressure back on the broken tailbone.
Now I'm in pretty much constant pain when I sit up; only lying down relieves it.
I tried the Donut Pillow trick. While it does take the stress off my tailbone, it makes me sit in a way that (you guessed it) strains my back in just the wrong place.
THIS is what comes of having a vertical support that, really, is designed to be a horizontal suspension bridge and that evolution has clumsily stopgapped into a sorta-acceptable solution.